Stepping motor, stepping motor device and driving method thereof

ABSTRACT

A stepping motor comprises a permanent magnet type rotor with a plurality of poles secured to a rotating shaft and a stator having stator magnetic poles with stator magnetic pole teeth in which excitation windings are wound on a plurality of magnetic poles in a star or delta connection, wherein the rotor is magnetized in different directions alternately circumferentially to satisfy the following equation: M=4F/3 where M is the number of poles of the rotor and F is the number of the stator magnetic poles, the rotor is cylindrical in shape with the stator rotatably disposed inside, disposed opposing the surfaces of the stator magnetic pole teeth through an air gap which is of a uniform dimension throughout the circumference between the surfaces of the stator magnetic pole teeth of the stator and the rotor, and the surface magnetic flux distribution thereof has a substantially sinusoidal wave form circumferentially. The stepping motor realizes smooth rotation and simplification in the structure of the rotor. In addition, the present invention provides a stepping motor device using the above-mentioned stepping motor and a method of driving the device.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a stepping motor structure, astepping motor device and a driving method thereof to rotate a rotarypolygon mirror for measuring the inter-vehicle distance, direction andrelative speed of a preceding vehicle.

[0003] 2. Description of Related Art

[0004] Conventionally, a stepping motor using a permanent magnet for arotor has been often used for driving a rotary portion of an officemachine such as a printer, high-speed facsimile equipment, a copyingmachine for PPC (Plain Paper Copier) or the like, since the steppingmotor has a high efficiency. Moreover, there has been a recently growinginterest in preventive safety technologies to prevent traffic accidentsfrom occurring by making vehicles more intelligent. One such preventivesafety technologies to be realized is driving environment recognition,specifically inter-vehicle distance control technology using laser radarand image recognition technology. A device for such a technologyrequires a laser scanner using a stepping motor and a rotary polygonmirror.

[0005] A two-phase stepping motor is mainly employed for use requiring amedium accuracy, while a three-phase stepping motor excellent in costperformance is employed for use requiring high-accuracy, low vibrationand low noise. As a stepping motor for office machines such as a laserprinter and facsimile equipment or the like requiring accuracy inpositioning and little unevenness in rotation, a three-phase machine hasbeen often employed in order to obtain high resolution and high torque.The three-phase machine comprises a cylindrical permanent magnet typerotor formed with multiple magnets in a cylindrical shape, or a hybridtype rotor having a permanent magnet held between two magnetic platesformed with multiple pole teeth, and a stator formed with pole teethopposite the rotor surface.

[0006] The stepping motor having the permanent magnet rotor wasaccurately step-driven one step angle at a time by a driving pulse inputfrom an outer part and an output shaft of the motor rotated as ifintermittently driven. Moreover, there was a growing tendency to userare earth magnets for the permanent magnet to be employed as a rotor inorder to obtain higher driving torque. The permanent magnet ismagnetized with different poles alternately in strips circumferentiallyand the magnet surface magnetic flux density measured along thecircumference of the magnet was shown as a substantially trapezoidaldistribution. Further, a trapezoidal or triangular surface of themagnetic pole teeth of a stator yoke is often employed in order toobtain high torque.

[0007] The three-phase machine provided with the cylindrical permanentmagnet type rotor or the hybrid type rotor and the stator formed withpole teeth is capable of obtaining high resolution and high torque asmentioned above. Since the distribution of the surface magnetic fluxdensity of the magnetized permanent magnet is substantially trapezoidalcircumferentially, step-like driving is easily achieved when the outputtorque is increased. On the other hand, it also has drawbacks such asincreased vibration upon driving and stopping the rotor and it makessmooth driving difficult. That is, noise or vibration is generated by avibration torque component contained in the torque generated by theproduct of excitation electric currents and field magnetic flux density.Accordingly, the above-mentioned construction wherein a large number ofharmonics are contained in the field magnetic flux density generated inan air gap between the permanent magnet of the rotor and the statorincreases noise and vibration.

[0008] Publications of Japanese Patent Application Nos. Hei 05-221388and Hei 09-325197 disclose a permanent magnet type stepping motor tolower damping of a rotor in starting or stopping a motor by solving suchproblems and rotating the rotor smoothly.

[0009] According to the publication of Japanese Patent Application No.Hei 05-221388, the permanent magnet of the rotor is skew-magnetized, themagnetic flux distribution in the magnetized surface of the permanentmagnet is made to be substantially sinusoidal circumferentially and themagnetic pole teeth of a stator yoke are made to be rectangular.

[0010] Moreover, according to the publication of Japanese PatentApplication No. Hei 09-325197, the relation between the number of statorpoles Q and the number of S-pole and N-pole pairs of the rotor N isQ=6k, and are set so as to satisfy N=yk(6n±1), and the magnetic poles ofthe stator are excited in a two-phase/three-phase excitation mode.

[0011] The stepping motor in accordance with each of the publications asmentioned above, however, has the following problems. Although themagnetized state of the rotor in a substantially sinusoidal shapecircumferentially is to lower vibration and noise in Japanese PatentApplication No. Hei 05-221388, the permanent magnet has an outerdiameter configuration in which a slightly uneven cross-section whichcontinues in each of the magnetic poles and is furthermoreskew-magnetized, which makes manufacturing difficult. Moreover, thepublication only discloses that a suitable number of magnetic poles(N-pole, S-pole) are skew-magnetized, but it does not describe thenumber of poles.

[0012] According to the publication Patent Application No. Hei05-221388, the relation between the number of the stator poles and thenumber of pairs of rotors is set so that the magnetic poles of thestator are excited in a two-phase/three-phase excitation mode; however,the magnetized state of the rotor is not described.

[0013] Further, when there is a steep load change of a rotating shaftand the rotation stops due to step out, the stepping motor according toeach of the above-mentioned publications remains in the stopped state.As a result, excessive current flows when the resistance value of statorwindings is small without back electromotive force being generated. Theflow of the excessive current increases the temperature of an adhesiveor the like fixing the windings, which may cause problems such as poorinsulation.

SUMMARY OF THE INVENTION

[0014] In view of the foregoing problems, it is an object of the presentinvention to provide a stepping motor having low vibration, a steppingmotor device which prevents the stepping motor from being damaged byrestarting in a case of a steep load change of a rotating shaft and therotation stops due to step out, and a driving method thereof.

[0015] In order to achieve the foregoing object, a stepping motoraccording to a first aspect of the present invention comprises apermanent magnet type rotor with a plurality of poles secured to arotating shaft and a stator having stator magnetic poles with statormagnetic pole teeth in which excitation windings are wound on aplurality of magnetic poles in a star or delta connection, wherein therotor is magnetized in different directions alternatelycircumferentially to satisfy the following equation: M=4F/3 where M isthe number of poles of the rotor and F is the number of the statormagnetic poles, the rotor is cylindrical in shape with the statorrotatably disposed inside, disposed opposing the surfaces of the statormagnetic pole teeth through an air gap which is of a uniform dimensionthroughout the circumference between the surfaces of the stator magneticpole teeth of the stator and the rotor, and the surface magnetic fluxdistribution thereof has a substantially sinusoidal wave formcircumferentially. This results in a stepping motor with smoothrotation. Also, in a conventional stepping motor, in order to make themagnetic flux distribution after magnetization approach a rectangularshape, a magnetic body such as iron is further provided on the magneticbody that has been magnetized to make the magnetic flux uniform.According to the present invention, however, a permanent magnet typerotor construction can be simplified by having the magnetic fluxmagnetized in a substantially sinusoidal shape circumferentially withoutproviding a magnetic body such as iron for making the magnetic fluxuniform.

[0016] In a preferred form according to the first aspect of the presentinvention, in the stepping motor, a cylindrical bearing holder isprovided for securing the rotating shaft in a predetermined location inan enclosure, the bearing holder vertically mounted by caulking to abase on which the stepping motor is mounted, and the rotor is disposedopposing the outside of the stator through the air gap which is of auniform dimension throughout the circumference between the rotor and thestator magnetic pole teeth surfaces and secured to the rotating shaftrotatably provided by a pair of bearings opposing one another throughthe bearing holder for securing the rotating shaft. As a result, astepping can be mounted to the base using a simple construction.

[0017] In another preferred form according to the first aspect of thepresent invention, in the stepping motor, the bearing holder has anarc-shaped deformation preventing groove to prevent deformation due tothe caulking. Accordingly, it can be prevented to occur mountingproblems such as the stepping motor detaching from the base after it hasbeen mounted thereon.

[0018] In still another preferred form according to the first aspect ofthe present invention, in the stepping motor, the arc-shaped deformationpreventing groove is provided along the circumference at the side endcontacting the base to which the stepping motor is mounted. Accordingly,occurrence of mounting problems such as the stepping motor detachingfrom the base due to external force from various directions after themotor has been mounted on the base can be prevented.

[0019] In yet another preferred form according to the first aspect ofthe present invention, in the stepping motor, the rotor is providedopposing the stator magnetic poles on a rotor yoke secured to therotating shaft and a notch is provided in the rotor yoke in order toleak magnetism of the rotor, and a leakage flux detector for detectingleaking magnetic flux from the rotor is provided in a position opposingthe notch. As a result, the stepping motor can detect stopping of thestepping motor due to a steep load change.

[0020] In another preferred form according to the first aspect of thepresent invention, in the stepping motor, a leakage flux detector fordetecting changes in magnetic poles is provided on a cylinder endsurface of a cylindrical permanent magnet provided in a cylindricalshape opposing the stator magnetic poles on the rotor yoke secured tothe rotating shaft. As a result, the stepping motor can detect thepositions of magnetic poles of the permanent magnet.

[0021] In yet another preferred form according to the first aspect ofthe present invention, in the stepping motor, a rotary polygon mirrorsecured to the rotating shaft which is rotatably provided through thecylindrical bearing holder vertically mounted on the base to which thestepping motor is mounted, which rotates along with the rotating shaft,is provided on the outer periphery of the rotor yoke with each mirrorsurface corresponding to a magnetic pole of the permanent magnet rotorof the stepping motor. As a result, a device for measuring theinter-vehicle distance, direction, and relative speed of a precedingvehicle can be made less expensive.

[0022] A stepping motor device according to a second aspect of thepresent invention comprises: a stepping motor including a permanentmagnet type rotor with a plurality of poles secured to a rotating shaft,a stator having stator magnetic poles with stator magnetic pole teeth inwhich excitation windings are wound on a plurality of magnetic poles ina star or delta connection, and a rotary polygon mirror provided on theouter periphery of a rotor yoke rotatable along with the rotating shaftwith each mirror surface corresponding to a magnetic pole of the rotor,wherein the rotor is magnetized in different directions alternatelycircumferentially to satisfy the following equation: M=4F/3 where M isthe number of poles of the rotor and F is the number of the statormagnetic poles, the rotor is cylindrical in shape with the statorrotatably disposed inside, disposed opposing to the surfaces of thestator magnetic pole teeth through an air gap which is of a uniformdimension throughout the circumference between the surfaces of thestator magnetic pole teeth of the stator and the rotor, and the surfacemagnetic flux distribution thereof has a substantially sinusoidal waveform circumferentially; a leakage flux detector for detecting changes inmagnetic poles provided on a cylinder end surface of the rotor of thestepping motor; a driving means to control rotation of the steppingmotor by impressing a driving signal in a three-phase single-two-phaseexcitation mode to three excitation feeding terminals in a star or deltaconnection wound on a plurality of magnetic poles of the stepping motor;and a means to detect the position of the rotary polygon mirror by asignal from the leakage flux detector. As a result, a device formeasuring the inter-vehicle distance, direction, and relative speed of apreceding vehicle is can be made less expensive.

[0023] A stepping motor device according to a third aspect of thepresent invention comprises: a stepping motor including a permanentmagnet type rotor with a plurality of poles secured to a rotating shaft,and a stator having stator magnetic poles with stator magnetic poleteeth in which excitation windings are wound around a plurality ofmagnetic poles in a star or delta connection, wherein the rotor ismagnetized in different directions alternately circumferentially tosatisfy the following equation: M=4F/3 where M is the number of poles ofthe rotor and F is the number of the stator magnetic poles, the rotor iscylindrical in shape with the stator rotatably disposed inside, disposedopposing the surfaces of the stator magnetic pole teeth through an airgap which is of a uniform dimension throughout the circumference betweenthe surfaces of the stator magnetic pole teeth of the stator and therotor, and the surface magnetic flux distribution thereof has asubstantially sinusoidal wave form circumferentially; a driving means toimpress a driving signal in a three-phase single-two-phase excitationmode to three excitation feeding terminals and to control rotation ofthe stepping motor by a signal from a leakage flux detector fordetecting magnetic flux leaking from a notch provided in a rotor yoke;and a means to repeat the processing to control the rotation apredetermined number of times and to issue a warning when normalrotation is not obtained. As a result, the device can detect stopping ofthe stepping motor due to a steep load change, thereby preventing themotor from being damaged.

[0024] A method of driving a stepping motor device according to a fourthaspect of the present invention, the stepping motor device comprising: astepping motor including a permanent magnet type rotor with a pluralityof poles secured to a rotating shaft and a stator having stator magneticpoles with stator magnetic pole teeth in which excitation windings arewound around a plurality of magnetic poles in a star or deltaconnection, wherein the rotor is magnetized in different directionsalternately circumferentially to satisfy the following equation: M=4F/3where M is the number of poles of the rotor and F is the number of thestator magnetic poles, the rotor is cylindrical in shape with the statorrotatably disposed inside, disposed opposing the surfaces of the statormagnetic pole teeth through an air gap which is of a uniform dimensionthroughout the circumference between the surfaces of the stator magneticpole teeth of the stator and the rotor, and the surface magnetic fluxdistribution thereof has a substantially sinusoidal wave formcircumferentially; and a driving means to impress a driving signal in athree-phase single-two-phase excitation mode to three excitation feedingterminals and to control rotation of the stepping motor by a signal froma leakage flux detector which detects magnetic flux leaking from a notchprovided in a rotor yoke, comprising the steps of: driving the steppingmotor by impressing the driving signal in the three-phasesingle-two-phase excitation mode to the three excitation feedingterminals in a star or delta connection wound on a plurality of magneticpoles of the stepping motor, detecting the signal from the leakage fluxdetector and comparing the changing speed of the signal of the leakageflux detector with the driving signal of the stepping motor, stoppingsupply of the driving signal of the stepping motor when there is adifference equal to or greater than a certain value in the comparisonresults, supplying the driving signals again after a predetermined time,repeating stopping and supplying processes of the driving signal for apredetermined number of times; and issuing a warning when normalrotation is not obtained. Accordingly, reliability of the device formeasuring the inter-vehicle distance, direction and relative velocity ofa preceding vehicle can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the accompanying drawings:

[0026]FIG. 1 is a side sectional view showing an embodiment of astepping motor of the present invention;

[0027]FIGS. 2A and 2B show the stepping motor in FIG. 1 viewed from top,in which FIG. 2A is a top plan view thereof and FIG. 2B is an enlargedview of part B in FIG. 2A;

[0028]FIGS. 3A, 3B and 3C explain a rotor yoke of a stepping motor ofthe present invention, in which FIG. 3A is a sectional view in the PRdirection, FIG. 3B is a plan view and FIG. 3C is a view showing the sidewith a notch 8 viewed from the QK direction;

[0029]FIGS. 4A, 4B, 4C, 4D and 4E show a bearing holder, in which FIG.4A is a bottom plan view in the B direction in FIG. 4B, FIG. 4B is apartially sectional view in the A direction in FIG. 4A, FIG. 4C is anenlarged view of part B in FIG. 4B, FIG. 4D is a view showingdeformation of a caulk portion in the case where a deformationpreventing groove is provided, and FIG. 4E is a view showing deformationof the caulk portion in the case where the deformation preventing grooveis not provided;

[0030]FIGS. 5A and 5B explain the relation between a permanent magnetand stator magnetic poles, in which FIG. 5A shows arrangement of thepermanent magnet and the stator magnetic poles and FIG. 5B is a viewshowing a magnetized state of the permanent magnet;

[0031]FIG. 6 is a block diagram of an embodiment of a stepping motordevice in the present invention;

[0032]FIGS. 7A, 7B and 7C explain the relation between a driving signaland current in the case where a stepping motor is driven in three-phasesingle-two-phase excitation mode, in which FIG. 7A is a diagram showingthe relation with an excitation signal in each step, FIG. 7B shows adriving signal in each step and FIG. 7C is a diagram showing changes incurrent flowing in a stator winding in each step;

[0033]FIGS. 8A to 8F are diagrams showing how a rotor rotates at thetime of excitation by the driving signal shown in FIGS. 7A, 7B and 7C,in which FIG. 8A to 8F correspond to each of the steps in FIGS. 7 A, 7Band 7C, respectively; and

[0034]FIG. 9 shows the relation between the rotary magnetic field andthe rotor on the excitation of the stator windings of the stepping motorin the present invention, in which {circle over (1)} to {circle over(6)} correspond to each of the steps in FIGS. 7 A, 7B and 7C,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Hereinafter, embodiments of the present invention will bedescribed with reference to the drawings.

[0036]FIG. 1 is a side sectional view of an embodiment of a steppingmotor according to the present invention. A column-like rotating shaft 1is rotatably provided to a cylindrical bearing holder 14 throughbearings 12 a and 12 b. The bearing holder 14 is vertically mounted on aplate 11 by a method described later with a spacer 16 at the outerperiphery of the bearing holder 14. A protruding portion 101 is providedinside of the bearing holder 14 so that the upper bearing 12 a does notfall. Moreover, a spring 13 is compressed to be inserted in between theprotruding portion 101 and the lower bearing 12 b. A ring 102 isprovided to the lower part of the rotating shaft 1 so that the bearing12 b does not fall. The spring 13 is compressed to be inserted inbetween the protruding portion 101 and the lower bearing 12 b, wherebygenerated repulsive force holds the bearing 12 b downward so that therotating shaft 1 does not move up and down.

[0037] The spacer 16 is provided with stator magnetic poles 2, forexample, made of laminated silicon steel plate or the like in thestructure described later fitted to the bearing holder 14 which issupported so as not to fall downward by the spacer 16. The statormagnetic pole 2 is wound by a stator winding 3 described later. Thestator winding 3 is led out to the outer portion by a connector 10through a driver circuit and a drive circuit (not shown) provided on aprinted circuit board 17 in connection with pins 15 and connected with amicrocomputer (MPU) for control (not shown). The pins 15 are connectedto four pieces in total: three-phase stator windings 3 and a neutralpoint of the stator windings 3 as described later.

[0038] A rotor yoke 4 of a magnetic body fitted on a bush 19 made of anonmagnetic body such as stainless steel provided and secured to therotating shaft 1 is rotatably provided opposing the stator magneticpoles 2. A permanent magnet 5, for example, made of a rare earth magnetor the like is provided along a circumference between the statormagnetic poles 2 and the rotor yoke 4 at the side where they face oneanother. And an air gap which is of a uniform dimension is providedbetween the permanent magnet and stator magnetic pole teeth (not shown)provided at the stator magnetic poles 2. The rotor yoke 4 and thepermanent magnet 5 are secured by means of an adhesive or the like.

[0039] The magnetic pole teeth of the stator magnetic poles have astructure along the circumference of the circle as shown in FIG. 5 indetail; therefore, a peripheral shape of the permanent magnet 5 providedwith the air gap which is of a uniform dimension between the permanentmagnet 5 and the stator magnetic pole teeth is also circular.Accordingly, the permanent magnet 5 is in a cylindrical shape with thestator inside of it. Incidentally, the positions of the rotor and thestator may be reversed. That is, the permanent magnet 5 to be the rotormay be inside and the stator magnetic poles to be the stator may beoutside thereof.

[0040] The permanent magnet 5 is magnetized with alternately differentpolarities circumferentially from the cylindrical magnetic body. Thesurface magnetic flux distribution thereof has a substantiallysinusoidal wave form circumferentially. The magnetization is performedby inserting an electromagnet, which is arc-shaped and which has apredetermined number of magnetized poles wound by windings, along theinternal portion of the cylindrical magnetic body and flowing a directcurrent.

[0041] A rotary polygon mirror 6 is provided on upper portion of thebush 19, which is secured by fitting in a ring 100 provided on therotating shaft 1 with 6 metal pieces 7 a and 7 b (others not shown)provided on upper part of the rotary polygon mirror 6. The rotarypolygon mirror 6 is provided with a protruding portion 110 for adjustingthe position of the bush 19. Positioning of the rotary polygon mirror 6with the rotor yoke 4 is carried out by fitting the bush 19 in a bore111 thereon.

[0042] A positioning groove 22 is aligned with a positioning groove (notshown) provided in the permanent magnet 5 so as to position thepermanent magnet 5 and the rotor yoke 4, thereby positioning of therotary polygon mirror 6, the rotor yoke 4 and the permanent magnet 5.

[0043] A leakage flux detector 9 comprising a hall element to detectmagnetic flux leaking from the permanent magnet 5 is provided on thelower part where the permanent magnet 5 rotates, which detects thestrength of the magnetic pole changing in accordance with the rotationof the permanent magnet 5. As mentioned above, the positions of therotary polygon mirror 6 and the permanent magnet 5 are set, thereby theposition of the rotary polygon mirror 6 is found by detecting the outputof the leakage flux detector 9.

[0044] The printed circuit board 17 is secured to the plate 11 by a bolt18 a and a nut 18 b.

[0045]FIGS. 2A and 2B show the stepping motor in FIG. 1 viewed fromabove, in which FIG. 2A is a top plan view thereof and FIG. 2B is anenlarged view of part B in FIG. 2A. In FIG. 2A, the rotary polygonmirror 6 has six leaves of mirrors, however, the number of mirrors isnot limited thereto. The rotary polygon mirror 6 is secured by fittingsix metal pieces 7 a and 7 b (others not shown) or the like provided onthe upper part of the rotary polygon mirror 6 as mentioned in FIG. 1into the ring 100 provided on the rotating shaft 1 and into six notches(not shown) provided in the upper part of the rotary polygon mirror 6,respectively.

[0046] A driver circuit 21 b is provided on the printed circuit board 17and connected to the rotor windings 3 through a printed wiring (notshown) by a method described later. Moreover, the driver circuit 21 b isconnected to a drive circuit 21 a through the printed wiring (notshown). The drive circuit 21 a is connected to a connector 10 throughthe printed wiring (not shown).

[0047] The pins 15 to connect the rotary windings 3 comprise 15 a, 15 b,15 c and 15 d, which are connected to stator winding terminals U, V andW described later and a neutral point N of the rotor winding,respectively. Each of the pins 15 is connected to the driver circuit 21b through the printed wiring (not shown) provided on the printed circuitboard 17. Incidentally, to prevent accidents such as disconnection ofthe circuit, a short circuit or the like caused by the connecting partof the neutral point N of the stator winding swinging due to vibrationor the like, the pin 15 d is connected to the neutral point N of thestator winding, but is not connected to the driver circuit 21 b.

[0048] At a position opposite the permanent magnet 5, a leakage fluxdetector 9 a is provided which will be described later.

[0049] In FIG. 2B, the positioning groove 22 is aligned with apositioning groove 23 provided in the permanent magnet 5 so as toposition the permanent magnet 5 and the rotor yoke 4, therebypositioning the rotary polygon mirror 6, the rotor yoke 4 and thepermanent magnet 5.

[0050]FIGS. 3A, 3B and 3C are explanatory views of the rotor yoke 4 ofthe stepping motor according to the present invention, in which FIG. 3Ais a sectional view in the PR direction, FIG. 3B is a top plan view andFIG. 3C is a view showing the side with a notch 8 viewed from the QKdirection. The permanent magnet 5 is adhered to the inside of the rotoryoke 4. The notch 8 having a length which does not exceed that of a polein the rotational direction of the permanent magnet 5, for example,which is approximately 70% thereof, is provided in a part of the rotaryyoke 4 so that the magnetic flux of the permanent magnet 5 is easilyleaked. There may be a plurality of notches 8, not only one. Moreover,their may be a plurality of leakage flux detectors 9 a, not only one.

[0051] The notch 8 is provided aligned with a predetermined position ofthe magnetic pole at the maximum magnetic flux density. Such positioningis carried out as follows. The relative location of the positioninggroove 22, the positioning groove 23 and the notch 8 is set so that whenthe positioning groove 22 is aligned with the positioning groove 23provided in the permanent magnet 5, the notch portion 8 aligns with alocation of the maximum magnetic flux density at predetermined pole ofthe permanent magnet 5.

[0052] The leakage flux detector 9 a as shown in FIGS. 2A and 2B isprovided to a position opposite the notch 8. The leakage flux detector 9a detects the magnetic flux leaking from the permanent magnet 5 andfunctions as described later.

[0053]FIGS. 4A, 4B, 4C, 4D and 4E are views of the bearing holder 14, inwhich FIG. 4A is a bottom plan view from the B direction in FIG. 4B,FIG. 4B is a segmental sectional view of the bearing holder 14 viewedfrom the A direction in FIG. 4A, FIG. 4C is an enlarged view of part Bin FIG. 4B, FIG. 4D is a view showing deformation of a caulk portion 44in the case where an arc-shaped groove 42 is provided, and FIG. 4E is aview showing deformation of the bottom of the bearing holder 14 in thecase where the groove 42 is not provided.

[0054] The bearing holder 14 is in a cylindrical shape as shown in FIG.4A. The protruding portion 101 provided inside the cylinder secures thebearing 12 b by repulsive force of the spring 13.

[0055] As shown in FIG. 4B, the ring 100 securing six metal piecesprovided on the upper portion of the rotary polygon mirror 6 and aconstricted part 43 to be fitted to a plate 11 are provided at the upperpart and at the lower part, respectively, throughout the outercircumference of the bearing holder 14. Moreover, the caulk portion 44and an arc-shaped groove 42 for preventing deformation due to caulkingare provided at the bottom of the bearing holder 14 throughout thecircumference.

[0056] The bearing holder 14 is inserted in a mounting hole (not shown)which is provided in a plate 11 and vertically mounted on the plate 11by being fitted as follows. The bearing holder 14 inserted into themounting hole (not shown) provided in the plate 11 is fitted to theplate 11 by applying pressure to the caulk portion 44 provided on thebottom in the arrow B direction of FIG. 4B, that is, in the direction ofthe cylinder of the bearing holder 14. The caulk portion 44 to which thepressure is applied is deformed outside of the cylinder as shown in FIG.4D.

[0057] In the outwardly deformed caulk portion 44, the constricted part43 to be fitted to the plate 11 and the deformed caulk portion 44sandwich the plate 11 to secure the bearing holder 14 to the plate 11.Moreover, at an inside 45 of the cylinder, warp which is absorbed in thearc-shaped groove 42 for preventing the deformation does not extend tothe inside 45 of the bearing holder 14. When the arc-shaped groove 42for preventing the deformation is not provided, a caulk mark 421 is leftby the pressure applied in the cylindrical direction of the bearingholder 14 and warp is caused and deforms the inside 45 of the bearingholder 14 as shown in FIG. 4E. The warp and the deformation lower thestrength of the bearing holder 14 as well as the fitting force betweenthe bearing holder 14 and the plate 11.

[0058]FIGS. 5A and 5B are explanatory views of the relation between thepermanent magnet 5 and the stator magnetic poles 2, in which FIG. 5Ashows arrangements of the permanent magnet 5 and the stator magneticpoles 2, and FIG. 5B is a view showing the magnetizing state of thepermanent magnet 5.

[0059]FIG. 5A shows the number of poles of the permanent magnets 5 to betwelve (six pairs of N poles and S poles) and the number of statormagnetic poles 2 being nine. However, the numbers may be other thanthese as long as the relation between the permanent magnet 5 and thestator magnetic poles 2 is satisfied as described later.

[0060] The stator magnetic poles 2 comprise magnetic pole teeth 2 aU, 2aV and 2 aW and magnetic pole pillars 2 bU, 2 bV and 2 bW on whichstator windings (not shown) are wound as described later. Moreover, anair gap wherein the space between the surfaces of the stator magneticpoles 2 and the permanent magnet 5 is of a uniform dimension throughoutthe circumference is provided at a surface opposite the permanent magnet5 of the magnet teeth 2 aU, 2 aV and 2 aW.

[0061] In the magnetized state of the permanent magnet 5 of the rotor,as shown in FIG. 5B, the magnetic flux distribution on the surface has asubstantially sinusoidal wave form circumferentially. In other words,changes from S pole to N pole by the rotation of the permanent magnet 5modifies magnetic force in a substantially sinusoidal wave form as shownin FIG. 5B.

[0062]FIG. 6 is a block diagram of an embodiment of a stepping motordevice according to the present invention. The stator winding terminalsU, V and W of a stepping motor 60 are connected to output terminals UO,VO and WO of the three-phase driver circuit 21 b, respectively. Outputterminals C1, C2 and C3 of the drive circuit 21 a and output terminalsCU, CV and CW of an MPU (microcomputer) 62 that is a driving means tocontrol the rotation of the stepping motor 60 are connected to inputsUIN, VIN and WIN of the three-phase driver circuit 21 b and inputterminals S1, S2 and S3 of the drive circuit 21 a, respectively.Moreover, output of the leakage flux detectors 9 a and 9 b comprising ahall element is connected to input terminals CH and CQ of the MPU 62through signal wires 63 and 64. Incidentally, power source circuits ofthe driver circuit 21 b, the drive circuit 21 a, the MPU 62 and theleakage flux detectors 9 are not shown. Moreover, a predeterminedprogram (not shown) in the MPU (microcomputer) 62 is saved and carriesout the processing of the stepping motor device as mentioned later.

[0063] The drive circuit 21 a carries out switching of each phase toexcite the stator winding by a predetermined pulse signal. A signal forgenerating a signal in a three-phase single-two-phase excitation modethat is a well-known driving method of stepping motor by the drivecircuit 21 a is outputted from the output terminals CU, CV and CW of theMPU 62. Moreover, stator windings 3U, 3V and 3W of the stepping motor 60are in a star connection in FIG. 6; however, it may be anotherconnecting method such as a delta connection.

[0064] The stator windings 3U, 3V and 3W are wound on magnetic polepillars 2 bU, 2 bV and 2 bW shown in FIG. 5A, respectively, as follows.That is, the stator winding 3U is wound on the magnetic pole pillar 2bU, the stator winding 3V is wound on the magnetic pole pillar 2 bV, andthe stator winding 3W is wound on the magnetic pole pillar 2 bW, allwith an equal number of windings so as to generate magnetic flux in thesame direction by a three-phase single-two-phase excitation current, forexample, to generate a predetermined magnetic pole, e.g. N pole at asurface where magnetic pole teeth 2 aU, 2 aV and 2 aW oppose thepermanent magnet 5 by a current flowing to the respective statorwindings.

[0065]FIGS. 7A, 7B and 7C explain the relation between a driving signaland a current in the case of driving the stepping motor 60 in thethree-phase single-two-phase excitation mode, in which FIG. 7A is a viewshowing the relation of the exciting signals in each step, FIG. 7B is aview showing the driving signals in each step, and FIG. 7C is a viewshowing changes of currents IU, IV and IW flowing to the stator windings3U, 3V and 3W in each step. Regarding the currents IU, IV and IW, thecurrents flowing from the terminals U, V and W of the stator windings3U, 3V and 3W are positive and the currents flowing out are negative.The currents flowing from the terminals in FIG. 7C are indicated as asingle units of current.

[0066] On driving in the three-phase single-two-phase excitation mode, ahalf cycle is completed in six steps as shown in FIG. 7A. As shown inFIG. 7A, when the stepping motor 60 is driven in the three-phasesingle-two-phase excitation mode, one or two phases of driving signalsare outputted alternately to each of the stator windings 3U, 3V and 3Wof the stepping motor 60. In other words, as shown in FIG. 7B, thedriving signal is outputted only to an output terminal UO of the drivercircuit 21 b in step 1, the driving signal is outputted to the outputterminals UO, VO of the driver circuit 21 b in step 2 and the drivingsignal is outputted only to the output terminal VO of the driver circuit21 b in step 3.

[0067] When the driving signals are impressed to each of the statorwindings 3U, 3V and 3W with the timing as shown in FIG. 7A, currents IU,IV, and IW flowing in each of the windings become as shown in FIG. 7C.In other words, when impedance of each winding is equally 2Z, thesynthesized impedance between the terminals respectively becomes 3Zwhatever the driving signal impressed to each terminal.

[0068] For example, in step 1, when the driving signals are impressedonly to the stator winding 3U and the other winding terminals aregrounded, current IU flows from terminal U to the winding havingimpedance 2Z and divides into {fraction (1/2)} to flow in the statorwindings 3V and 3W of impedance 2Z, respectively. Then, in step 2, whenthe driving signal is impressed to the stator windings 3U and 3V,{fraction (1/2)} of the current flowing from the stator winding 3U and{fraction (1/2)} of the current flowing from the stator winding 3V mergein the stator winding 3W to be IW and flow out from the terminal W.Similarly, a current of each winding in each step becomes as shown FIG.7C. As clear from FIG. 7C, the current flowing in each winding does notbecome a rectangular shaped current at each phase, but becomes astair-shaped current. The rotor mutually working with the magnetic fluxdistribution of the permanent magnetic type rotor is magnetized in asubstantially sinusoidal wave form to be smoothly driven.

[0069]FIGS. 8A to 8F are views explaining how the rotor is rotated whenthe stator winding is excited with driving signals of the three-phasesingle-two-phase excitation mode as shown in FIGS. 7A, 7B and 7C, inwhich FIG. 8A to 8F correspond to each step in FIGS. 7A, 7B and 7C,respectively. Incidentally, in FIGS. 8A to 8F the magnetic poles of therotor comprise two poles of only an N pole and an S pole for simplifyingthe description; the others are not shown.

[0070] In FIG. 8(a), when the driving signals are impressed only to thestator winding 3U in step 1, a current IU flows from terminal U and themagnetic pole N is generated in the direction between the statorwindings 3V and 3W by the current. Similarly, the magnetic pole S isgenerated to be in the direction between the stator windings 3U and 3Vby a current IW flowing from the stator winding 3W. Similarly a magneticpole N is generated by a current IV flowing from the stator winding 3Vand the magnetic pole S is generated so as to be in the directionbetween the stator windings 3U and 3W. Synthesized magnetic fields ofthe S pole and N pole generated by each winding become ST and NT asrespectively shown.

[0071] Similarly, when the driving signals are impressed to the statorwindings 3U and 3V in step 2, {fraction (1/2)} of the current flows fromthe stator winding 3U and the remaining {fraction (1/2)} flows from thestator winding 3V, respectively, to merge in the stator winding 3W as IWand flow out of the terminal W. The synthesized magnetic fields of the Spole and N pole as mentioned above are rotated 60 degrees to the right,as shown in FIG. 8B and become ST and NT by the current. Similarly, thesynthesized magnetic fields ST and NT are rotated in increments of 60degrees to the right so as to rotate in a complete circle in six steps.The magnetic pole of the rotor is drawn to the rotating magnetic fieldby the stator winding and rotates in a complete circle in 6 steps. Itshould be noted that FIGS. 8C to 8F do not show the synthesized magneticfield NT.

[0072]FIG. 9 shows the relation between the rotor and the rotatingmagnetic field at the time of excitation of the stator winding in anembodiment of the present invention, in which {circle over (1)} to{circle over (6)} correspond to each step of FIG. 7A, respectively. InFIG. 9, the number of magnetic poles U, V and W of the stator 90 isthree, respectively, the number of the poles is nine in total and thenumber of the magnetic poles of the rotor 91 is twelve. A driving signalis impressed to each winding (not shown) with the timing shown in FIG.7A in a manner shown as {circle over (1)} to {circle over (6)} in FIG.9. In such a case, the rotation angle in one step is {fraction (1/12)}compared to FIGS. 8A to 8F. The rotor 91 rotates by one magnetic pole inthree steps and rotates in a complete circle smoothly in thirty-sixsteps.

[0073] The number of poles is determined as follows.

[0074] When the number of stator magnetic poles is F, the number ofpoles M of the permanent magnet type rotor is determined as follows.That is, in case of driving a stepping motor in three-phase, the numberof stator magnetic poles F is a multiple of 3. Since a rotor is providedwith a multi-pole rotating magnet having two poles of S and N, thenumber of pole pairs M/2 of the rotor is a multiple of 2. Accordingly,the number of the stator magnetic poles F is a multiple of 3, and thereare various combinations with the number of the stator magnetic poles F,which are a multiple of 2 of the number of the pole pairs M/2 of thepermanent magnet type rotor. In the present invention, the relation ofthe number of the poles of the permanent magnet type rotor M and thenumber of the stator magnetic poles F is set so as to satisfy M=4F/3 sothat the permanent magnet type rotor magnetized in a sinusoidal waveform rotates smoothly.

[0075] The movement of a stepping motor device according to the presentinvention will be illustrated with reference to FIG. 6. The drivecircuit 21 a generates a signal in a three-phase single-two-phaseexcitation mode which is a well-known method of driving a steppingmotor, and a series of three-phase signals as shown in FIG. 7A areconsecutively outputted from the output terminals C1, C2 and C3. Thedriving signals are impressed to the input UIN, VIN and WIN terminals ofthe driver circuit 21 b. The drive circuit 21 a and the driver circuit21 b are realized by a well-known semiconductor integrated circuitavailable on the market. The impressed driving signals to input aretransformed into three-phase currents IU, IV and IW. The MPU 62generates a signal for actuating the drive circuit 21 a by a well-knownmethod.

[0076] The three-phase currents IU, IV and IW flow to the statorwindings 3U, 3V and 3W from the terminals U, V and W of the steppingmotor 60 and the stepping motor 60 is rotated at a predetermined speed.

[0077] A driving method of the stepping motor device of the presentinvention which operates as described above will hereinafter bedescribed. A leakage flux detector 9 a provided opposing a notch 8provided in a part of a rotor yoke 4 detects magnetic flux leaking froma permanent magnet 5 provided on the rotor yoke 4. When the steppingmotor 60 rotates, a detecting signal synchronized with the rotation ofthe rotor yoke 4 is obtained at an input terminal CH of the MPU 62connected with the output of the leakage flux detector 9 a. Thedetecting signal changes to positive and negative by the leakage fluxwherein the N pole and S pole of the permanent magnet 5 are generatedalternately. Accordingly, the MPU 62 detects the signal of the inputterminal CH and the detected signal changing to positive and negative ata predetermined speed means normal rotation.

[0078] In case of some problems, for example, if the stepping motor 60stops rotating due to a steep increase in load, the output of theleakage flux detector 9 a does not change to positive and negative and acertain value or a changing speed is lowered. When the output changingspeed of the leakage flux detector 9 a and output signals to the drivecircuit 21 a driving the stepping motor are compared and there is adifference equal or less than a certain value, the MPU 62 stopssupplying the driving signal. Then after a predetermined time, itsupplies the driving signal again. The MPU 62 repeats the supply of thedriving signal for a predetermined number of times. When the rotation ofthe stepping motor 60 is not normal, the MPU 62 issues a notification ofmalfunctioning to a lamp (not shown) or a system to which the steppingmotor 60 is mounted.

[0079] The leakage flux detector 9 b detects magnetic flux coming out ofthe lower part of the rotor yoke 4. The position of the rotor yoke 4 isset in relation to the rotary polygon mirror 6 as mentioned above.Accordingly, a relative position of the rotary polygon mirror 6 can befound by detecting the changes of the magnetic flux of the rotor yoke 4.The MPU 62 inputs the output of the leakage flux detector 9 b via asignal line 64 to find the position of the rotary polygon mirror 6 andoutputs to a device (not shown) for measuring the inter-vehicledistance, direction, and relative speed of a preceding vehicle.

[0080] Since the position of the rotary polygon mirror 6 where theoutput of the leakage flux detector 9 a changes is fixed, the absoluteposition of the rotary polygon mirror 6 can be found by processing thisalong with the output of the leakage flux detector 9 b.

What is claimed is:
 1. A stepping motor comprising: a permanent magnettype rotor with a plurality of poles secured to a rotating shaft and astator having stator magnetic poles with stator magnetic pole teeth inwhich excitation windings are wound on a plurality of magnetic poles ina star or delta connection, wherein the rotor is magnetized in differentdirections alternately circumferentially to satisfy the followingequation: M=4F/3 where M is the number of poles of the rotor and F isthe number of the stator magnetic poles, the rotor is cylindrical inshape with the stator rotatably disposed inside, disposed opposing thesurfaces of the stator magnetic pole teeth through an air gap which isof a uniform dimension throughout the circumference between the surfacesof the stator magnetic pole teeth of the stator and the rotor, and thesurface magnetic flux distribution thereof has a substantiallysinusoidal wave form circumferentially.
 2. A stepping motor according toclaim 1, further comprising a cylindrical bearing holder for securingthe rotating shaft in a predetermined location in an enclosure, thebearing holder vertically mounted by caulking to a base on which thestepping motor is mounted, wherein the rotor is disposed opposing theoutside of the stator through the air gap which is of a uniformdimension throughout the circumference between the rotor and the statormagnetic pole teeth surfaces and secured to the rotating shaft rotatablyprovided by a pair of bearings opposing one another through the bearingholder.
 3. A stepping motor according to claim 2, wherein the bearingholder has an arc-shaped deformation preventing groove to preventdeformation due to the caulking.
 4. A stepping motor according to anyone of claims 1 to 3, wherein the arc-shaped deformation preventinggroove is provided along the circumference at the side end contactingthe base to which the stepping motor is mounted.
 5. A stepping motoraccording to any one of claims 1 to 4, wherein the rotor is providedopposing the stator magnetic poles on a rotor yoke secured to therotating shaft and a notch is provided in the rotor yoke in order toleak magnetism of the rotor, further comprising a leakage flux detectorfor detecting leaking magnetic flux from the rotor, the leakage fluxdetector provided in a position opposing the notch.
 6. A stepping motoraccording to any one of claims 1 to 5, further comprising a leakage fluxdetector for detecting changes in magnetic poles, the leakage fluxdetector provided on a cylinder end surface of a cylindrical permanentmagnet provided in a cylindrical shape opposing the stator magneticpoles on the rotor yoke secured to the rotating shaft.
 7. A steppingmotor according to claim 5 or 6, further comprising a rotary polygonmirror secured to the rotating shaft which is rotatably provided throughthe cylindrical bearing holder vertically mounted on the base to whichthe stepping motor is mounted, which rotates along with the rotatingshaft, the rotary polygon mirror provided on the outer periphery of therotor yoke with each mirror surface corresponding to a magnetic pole ofthe rotor of the stepping motor.
 8. A stepping motor device comprising:a stepping motor including a permanent magnet type rotor with aplurality of poles secured to a rotating shaft, a stator having statormagnetic poles with stator magnetic pole teeth in which excitationwindings are wound on a plurality of magnetic poles in a star or deltaconnection, and a rotary polygon mirror provided on the outer peripheryof a rotor yoke rotatable along with the rotating shaft with each mirrorsurface corresponding to a magnetic pole of the rotor, wherein the rotoris magnetized in different directions alternately circumferentially tosatisfy the following equation: M=4F/3 where M is the number of poles ofthe rotor and F is the number of the stator magnetic poles, the rotor iscylindrical in shape with the stator rotatably disposed inside, disposedopposing the surfaces of the stator magnetic pole teeth through an airgap which is of a uniform dimension throughout the circumference betweenthe surfaces of the stator magnetic pole teeth of the stator and therotor, and the surface magnetic flux distribution thereof has asubstantially sinusoidal wave form circumferentially; a leakage fluxdetector for detecting changes in magnetic poles provided on acylindrical end surface of the rotor of the stepping motor; a drivingmeans to control rotation of the stepping motor by impressing a drivingsignal in a three-phase single-two-phase excitation mode to threeexcitation feeding terminals in a star or delta connection wound on aplurality of magnetic poles of the stepping motor; and a means to detectthe position of the rotary polygon mirror by a signal from the leakageflux detector.
 9. A stepping motor device comprising: a stepping motorincluding a permanent magnet type rotor with a plurality of polessecured to a rotating shaft, and a stator having stator magnetic poleswith stator magnetic pole teeth in which excitation windings are woundaround a plurality of magnetic poles in a star or delta connection,wherein the rotor is magnetized in different directions alternatelycircumferentially to satisfy the following equation: M=4F/3 where M isthe number of poles of the rotor and F is the number of the statormagnetic poles, the rotor is cylindrical in shape with the statorrotatably disposed inside, disposed opposing the surfaces of the statormagnetic pole teeth through an air gap which is of a uniform dimensionthroughout the circumference between the surfaces of the stator magneticpole teeth of the stator and the rotor, and the surface magnetic fluxdistribution thereof has a substantially sinusoidal wave formcircumferentially; a driving means to impress a driving signal in athree-phase single-two-phase excitation mode to three excitation feedingterminals and to control rotation of the stepping motor by a signal froma leakage flux detector for detecting magnetic flux leaking from a notchprovided in a rotor yoke; and a means to repeat the processing tocontrol the rotation a predetermined number of times and to issue awarning when normal rotation is not obtained.
 10. A method of driving astepping motor, the stepping motor including a permanent magnet typerotor with a plurality of poles secured to a rotating shaft and a statorhaving stator magnetic poles with stator magnetic pole teeth in whichexcitation windings are wound around a plurality of magnetic poles in astar or delta connection, wherein the rotor is magnetized in differentdirections alternately circumferentially to satisfy the followingequation: M=4F/3 where M is the number of poles of the rotor and F isthe number of the stator magnetic poles, the rotor is cylindrical inshape with the stator rotatably disposed inside, disposed opposing thesurfaces of the stator magnetic pole teeth through an air gap which isof a uniform dimension throughout the circumference between the surfacesof the stator magnetic pole teeth of the stator and the rotor, and thesurface magnetic flux distribution thereof has a substantiallysinusoidal wave form circumferentially; and a driving means to impress adriving signal in a three-phase single-two-phase excitation mode tothree excitation feeding terminals and to control rotation of thestepping motor by a signal from a leakage flux detector which detectsmagnetic flux leaking from a notch provided in a rotor yoke, the methodcomprising the steps of: driving the stepping motor by impressing thedriving signal in the three-phase single-two-phase excitation mode tothe three excitation feeding terminals in a star or delta connectionwound on a plurality of magnetic poles of the stepping motor, detectingthe signal from the leakage flux detector and comparing the changingspeed of the signal of the leakage flux detector with the driving signalof the stepping motor, stopping supply of the driving signal of thestepping motor in case of a difference equal to or greater than acertain value in the comparison results, supplying the driving signalsagain after a predetermined time, repeating stopping and supplyingprocesses of the driving signal for a predetermined number of times; andissuing a warning when normal rotation is not obtained.